How to Play Co-Op – Stardew Valley
The official multiplayer for Stardew Valley is slated to drop sometime next year. Until then, you can download Spacechase0’s mod titled Makeshift Multiplayer. The intention of the mod is to be a way for players to have a co-op in the time before multiplayer gets officially added to the game, so the creator states that certain bugs and crashes users face may go ignored.
Once the mod is downloaded, load the game like usual and a pop-up will appear that gives players the option to play as a host, client or single-player. If you are a client, you cannot play at all until the host starts the game.
In the co-op mode, inventory, wallet, skills etc. will be shared (with the exception being your house). In-game chat can be accessed by pressing the enter key and everyone needs to sleep in order to trigger the next day cycle.
You can download the mod here.